1. Field of the Invention
The present invention relates to a sheet processing apparatus that performs a predetermined process on a sheet, such as a paper sheet, and to an image forming system that includes the sheet processing apparatus.
2. Description of the Related Art
Conventionally, as an image forming system, one that includes a sheet processing apparatus that is able to sort sheets on which images have been generated by an image forming apparatus has been know.
A sheet processing apparatus described in Japanese Patent No. 4063704 includes: a paper ejection tray on which paper sheets, which are sheets ejected from an outlet of a main body of the apparatus, are to be loaded; and a pair of paper ejection rollers, which eject the paper sheets to the paper ejection tray and in which a pair of roller members are configured to be able to come into contact with and separate from each other. Further, a pair of shift rollers are included, which are provided upstream of the pair of paper ejection rollers in a sheet conveyance direction, are movable in a paper sheet width direction, convey the paper sheets towards the paper ejection tray, and perform a shift operation of shifting the paper sheets to different positions in the paper sheet width direction.
During the shift operation on the paper sheets by the pair of shift rollers, the pair of paper ejection rollers are separated from each other, and after the shift operation is completed, the pair of paper ejection rollers are abutted onto each other to convey the paper sheets while holding the paper sheets therebetween and eject the paper sheets to the paper ejection tray. When the paper sheets ejected by the pair of paper ejection rollers are loaded on the paper ejection tray, the paper sheets are distributed to left and right in paper the sheet width direction by a predetermined amount by the pair of shift rollers to load the paper sheets onto different shift positions on the paper ejection tray.
However, conventionally, controlling the shift operation has been carried out in the same way regardless of a type of the sheets, and thus, depending on the type of sheets, if the shift operation is controlled as described above, failure in the loading of the sheets may be caused. For example, if sheets not having enough firmness as sheets are used and the shift operation is controlled as described above, the following problems may be caused.
Specifically, since the paper sheets are conveyed while being shifted by the pair of shift rollers in a state in which the pair of paper ejection rollers are separated, a paper sheet leading end side is not held by the pair of paper ejection rollers and only a paper sheet tail end side is held by the pair of shift rollers, providing no firmness at the paper sheet leading end side. Therefore, the paper sheet leading end side not provided with firmness is largely hung down towards the paper ejection tray.
After the shift operation on the paper sheets by the pair of shift rollers is completed, the pair of paper ejection rollers are caused to come into contact with each other and the paper sheets are conveyed while being held between the pair of paper ejection rollers. However, if the paper sheets are conveyed by the pair of paper ejection rollers in a state in which the paper sheet leading end side not provided with firmness is largely hung down towards the paper ejection tray, when a sheet leading end contacts the paper ejection tray, there is a risk that curling may occur at the sheet leading end side and failure in the loading of the paper sheets may be caused.
In view of the above, there is a need to provide a sheet processing apparatus, which is able to suppress failure in loading of sheets that have been subjected to a shift operation, regardless of a type of the sheets, as well as an image forming system including the sheet processing apparatus.